1. Field of the Invention
The invention relates in general to static electrical inductive apparatus, such as electrical transformers, and more specifically to new and improved methods of constructing magnetic cores for such apparatus.
2. Description of the Prior Art
The core losses in the electrical transformers used by electric utility companies represent a significant loss of the energy generated, even though electrical transformers are highly efficient. With the increasing value of energy, ways of reducing these losses are constantly being sought. The use of amorphous metal in the magnetic cores of electrical power transformers appears to be attractive, because at equivalent inductions the no-load core losses of electrical grade amorphous metals are only about 25% to 30% of the losses of conventional grain-oriented electrical steels.
Amorphous metals, however, in addition to their higher initial cost than conventional electrical steels, also pose many manufacturing problems which are not associated with conventional grain-oriented steels. For example, amorphous metal is very thin, being only about 1 to 1.5 mils thick, it is very stress sensitive, with the losses and excitation power of cores constructed of amorphous metal both being adversely affected by mechanical stresses; and, it is very brittle, especially after stress-relief anneal. These characteristics create many manufacturing problems, especially in constructing magnetic cores of the stacked type. A large number of laminations must be stacked, even to reach a build of 1 inch, for example, making it very time consuming to stack power transformer cores, which usually have build dimensions of several inches. Further, the large number of laminations in the core build results in a relatively low space factor, compared with a core constructed of conventional grain oriented electrical steel. Amorphous laminations are not perfectly flat, nor are they are perfectly smooth. Amorphous laminations have ripples and dimples, as well as surface irregularities. These characteristics, along with the large number of lamination-to-lamination interfaces, cause the relatively low space factor. Clamping the amorphous core to increase the space factor applies stresses to the core, which in turn increase both the core losses and the exciting volt amperes required to magnetize the core.
The prior art has tried many different approaches to decrease the time required to stack a core using amorphous laminations, as well as to increase the space factor. Laminate composites, using polymers or metals having a low melting point to bond a plurality of laminations into a single lamination, make it easier to stack a core, but anything placed between the laminations reduces the space factor. Metallurgically bonding a plurality of amorphous laminations to create a composite lamination solves the problem of introducing a foreign substance between the laminations, but such a construction may increase eddy current losses, apparently because the beneficial effect of having a large number of thin laminations is partially lost by the metal-to-metal contact provided by the metallurgical bonds.
Thus, it would be desirable to provide a new and improved method of constructing magnetic cores for power transformers using amorphous alloys, which method would improve the core space factor while reducing the sensitivity of the core to the clamping stresses required to achieve and maintain a satisfactory space factor.